This invention relates to sheet handling drums and, more particularly, it concerns an unique sheet clamping arrangement by which the leading and trailing edges of a sheet fed from a supply tray or stack are clamped to a drum for the performance of a printing operation, for example, delivered from the drum to a delivery tray or stack, and the drum and clamping arrangement repositioned to receive another sheet.
The prior art relating to rotatable sheet handling drums and clamp-like retention devices for holding a sheet on a drum is highly developed in machinery involving a transfer of some media to or from the drum retained sheet. In many types of printing machines, for example, the drum retained sheet may control transfer of ink to a succession of receiver sheets whereas in various recording machinery, the drum retained sheet receives ink or its equivalent from a stylus or other writing device. In this latter type of machinery, the support of the receiving sheet by the drum permits accurate angular registration of the sheet with respect to a writing device usually supported by a carriage for movement axially of the drum.
The support provided by the cylindrical surface of a drum combined with the capability for accurate angular registration at relatively high speeds makes a rotatable drum support for receiver sheets an excellent candidate for use in computer driven electronically controlled printers. A major obstacle to this use of sheet retaining drums, however, is presented by problems associated with securement of the sheet to the drum periphery in a manner which does not crease, fold or otherwise deform the sheet and also in delivery of the sheet from the drum after a work cycle is completed.
U.S. Pat. Nos. 4,033,575 and 4,259,695 contain disclosures of clamping apparatus for retaining a sheet about the periphery of a rotatable drum and by which the leading and trailing edges of the sheet are successively clamped by independently movable clamping bars. In the disclosures of both of these patents, the leading edge of a sheet fed tangentially to the drum is first seized by a leading edge clamping bar rotatable at all times with the drum. While the trailing edge clamp is held against movement, the drum and the leading edge clamp draw the sheet past the trailing edge clamp until the trailing edge of the sheet registers with the latter clamp. The trailing edge clamp is then closed on the sheet and rotates with the drum during a sheet processing cycle.
The drum sheet clamping arrangements shown in the aforementioned patents, in principle, are desirable from the standpoint of providing a firm clamping action at opposite ends of a sheet, of enabling the clamping action to occur during drum rotation in one direction, and of avoiding any need for folding, creasing or otherwise mutilating the sheet. On the other hand, the mechanisms required for operation of the clamping apparatus disclosed in these patents are complicated and tend to restrict use of the apparatus to relatively slow-speed facsimile machines. In addition, the disclosed clamping apparatus does not operate by itself to discharge the sheet from the drum.
There is, therefore, a need for improvement in the drum sheet clamping apparatus heretofore disclosed in order to adapt rotatable drum sheet supports to machinery such as computer driven printers to and from which successive print receiving sheets are fed and discharged automatically.